1. Field of the Invention
The present invention relates to a vibration speaker mounted in a communication system generating acoustic sound and vibration, and more particularly, to a vibration speaker and a method of changing a spring strength of an elastic member to adjust an actual resonance frequency to a designed resonance frequency and to provide a stable vibration characteristic to the vibration speaker.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional micro speaker used in a mobile (portable) communication terminal. The micro speaker includes a case 100 having an internal space, a magnet 110 and a voice coil 120 mounted in the case 100, and a vibration plate 130 generating audio sound.
In the speaker, a current of a high frequency flows from an external source to the voice coil 120 through a lead line 101 to a magnetic field between the voice coil 120 and the magnet 110. The magnetic field causes the voice coil 120 to more upward and downward, and the vibration plate 130 coupled to the voice coil 120 vibrates to generate the audio sound.
Since the high frequency current supplied to the voice coil 120 through the lead line 101 is an alternative current, depending on a direction of the current flowing through the voice coil 120, and an attraction force is generated between the magnet 110 and the voice coil 120 if the magnetic field formed by the voice coil 120 and another magnetic field formed by the magnet 110 are the same direction. Accordingly, the voice coil 120 moves downward forward the magnet 110 due to the attraction force.
To the contrary, if the magnet field formed by the voice coil 120 and the another magnetic field formed by the magnet 110 are different directions, a repulsive force in generated between the magnet 110 and the voice coil 120, and the voice coil 120 is pushed upward away from the magnet 110.
As described above, the voice coil 120 moves upward and downward according to a change of the magnetic field forward by the voice coil 120 and causes the vibration plate 130 attached to the voice coil 120 to vibrate up and down to generate the audio sound.
The mobile telecommunication terminal is provided with a vibration unit (function) notifying a user of a receiving call using vibration other than the audio sound as well as an audio sound generator.
A vibration motor has been used as the vibration unit. However, it is a technical limitation in minimizing a size of the vibration motor depending on a trend of a slim mobile telecommunication terminal. Recently, a vibration speaker is adopted in the mobile telecommunication terminal as the vibration unit (function) together with the audio sound generator.
FIG. 2 is a cross-sectional view of a conventional vibration speaker. As shown in FIG. 2, the vibration speaker generates the audio sound and the vibration by selectively supplying a high frequency current or a low frequency current to a voice coil 120.
The conventional vibration speaker includes a case 100 forming an external shape and providing an inner space, and a yoke 105 disposed in the inner space of the case 100.
The yoke 105 is provided with a pair of plate springs 150, 155 mounted on inner upper and lower side portions of the case 100.
The plate springs 150, 155 include an outer circumferential side fixedly inserted into grooves 100 formed on the inner upper and lower side portions of the case 100, respectively.
A magnet 110 is mounted on an inner center portion of the yoke 105, and a vibration coil 115 is mounted below the magnet 110, that is, on an upper surface of a lower plate 102 of the case 100.
A vibration plate 130 is mounted on an upper portion of the case, and a voice coil 120 is extended from the vibration plate 130 toward the magnet 110.
In the vibration speaker having the above structure, a weight 140 is provided on an outer side of the yoke 105 to maximize an amount of the vibration, and the weight 140 is disposed between the plate springs 150, 155.
In the conventional vibration speaker having the above structure when a high frequency signal is inputted to the voice coil 120, the vibration plate 130 is minutely vibrated by an electromagnetic force formed among the voice coil 120, the magnet 110 and the vibration coil 115 to generate the audio sound. Accordingly, the vibration speaker can be used as the audio sound generator.
When a low frequency signal is inputted to the vibration coil 115, the yoke 105 moves upward and downward by the electromagnetic force generated between the vibration coil 115 and the magnet 110, and upward and downward movements of the yoke 105 are transmitted to the case 100 through the plate springs 150, 155 to perform the vibration function.
The vibration speaker moves a vibration member constituted of the yoke 105, the magnet 110 and the weight 140 to generate the vibration by harmonizing a resonance frequency of a product employing the vibration speaker with a predetermined frequency.
In the above conventional vibration speaker, a deviation in amounts of respective vibrations of the yoke 105, the magnet 110, and the weight occurs according to an assembly dispersion (variation) of the yoke 105, the magnet 110, and the weight 140, and also, another deviation between a designed resonance frequency and an inherent resonance frequency of the vibration member occurs by the difference in the amounts of the respective vibrations according to a measurement dispersion (variation) of respective parts.
In the assembly dispersion, assembling positions and amounts of attachment of the parts constituting the vibration speaker becomes different from designed ones. In the measurement dispersion, thickness, width, and length of constituents of the vibration speaker becomes different from the designed ones.
Particularly the measurement dispersion occurs mainly in the plate springs 150, 155 which are one of major factors to determine an amount of the vibration. The measurement dispersion of the plate springs 150, 155 occurs due to a small amount of a non-uniform thickness and a difference between an actual measurement and a designed measurement of the plate springs 150, 155.
The plate springs 150, 155 have a non-uniform thickness when the plate springs 150, 155 are manufactured from an original material, such as a steel plate. According to a current technology to make the steel plate, it is impossible to make a perfectly uniform steel plate.
It is also impossible to avoid the difference in forming the plate springs 150, 155 due to a physical limitation.
FIG. 3 is a graph showing a relationship between an amount of the vibration and the designed resonance frequency of the convention vibration speaker. A vibration speaker manufacturer makes the vibration speaker according to the designed resonance frequency requested by a user.
The resonance frequency is 182 Hz or 139 Hz according to characteristics of the telecommunication terminal mounted with the vibration speaker.
The graph of FIG. 3 is an example showing on of various resonance frequencies and the resonance frequency of 182 Hz and the amount of the vibration there from.
As shown in FIG. 3, the vibration speaker manufacturer designs and makes the vibration speaker according to the resonance frequency of 182 Hz. However, the manufactured vibration speaker generates the actual resonance frequency below or above 182 Hz due to the assembly deviation (deflection) and the manufacturing (processing) deviation.
The actual resonance frequency of the manufactured vibration speaker is generated in a region disposed on the right or left with respect of the designed resonance frequency of 182 Hz. As a result, the manufactured vibration speaker becomes a defected speaker having lower vibration characteristics and a lower or higher resonance frequency with respect to the designed resonance frequency.
The resonance frequency can be expressed by a formula, fn=½π (K/M)1/2, where in K is a strength of the plate springs 150, 155 (elastic unit), and M is a mass of the plate spring 150, 155 and the vibration member (vibrator).
As shown in the above formula, the resonance frequency fn is proportional to the strength K and the mass M which are determinants of the resonance frequency.
A general vibration speaker is designed to have the strength of 130 gf/mm and a total vibrator mass of 1.8 gf. According to changes of the strength of 2 gf/mm and the total vibrator mass of 0.03 gf, the resonance frequency is changed by 1 Hz.
Although the mass of 0.03 gf is a very minute amount compared to the total vibrator mass of 1.8 gf, the mass of 0.03 gf affects the resonance frequency very largely since the vibration amount of the vibrator drastically decreases according to a change of the resonance frequency by 2 or 3 Hz.
Also, the strength of 2 gf is a very minute amount compared to the strength of 130 gf. However, the minute amount of the strength by 2 gf affects the frequency very largely like as the change of the total vibrator mass by 0.03 gf. The strength of 2 gf corresponds to a thickness of 1 mm in the plate springs 150, 155.
It is almost impossible to generate the same resonance frequency as the designed resonance frequency of the vibration speaker if the strength and the total vibrator mass of the plate springs 150, 155 are changed even by a small amount.
When the strength and the total vibrator mass of the plate springs 150, 155 are changed, the vibrator cannot maintain a maximum effective vibration amount of 2.5 G, but reaches 3.5 G, and accordingly, an vibration amplitude increases, thereby, causing the vibrator to contact lower and upper surfaces of the case 100.
In order to provide the vibration with the designed resonance frequency as the actual resonance frequency, the strength of the respective parts and the plate springs 150, 155 constituting the vibration speaker must be maintained uniform in a manufacturing process. Accordingly, the manufacturing process should be managed with a very steep restriction on the strength.
However, a manufacturing cost of the vibration speaker increases in proportion to an increase of a parts manufacturing cost if the manufacturing process of the parts is strictly managed to maintain the strength of the parts and the plate springs 150, 155 uniform.
In a method of maintaining the strength and the mass of the plate springs 150, 155, the plate springs 150, 155 are managed to maintain the thickness 1˜2 mm. However, it is impossible to technically manage the uniform thickness of 1˜2 mm in the plate springs 150, 155.
Even if the strength and the mass of the parts and the plate springs. 150, 155 are maintained, and the parts are strictly managed in the manufacturing process, the defected vibration speaker having a different resonance frequency from the designed resonance frequency due to the assembly dispersion and the measurement dispersion as explained above. Thus, the vibration speaker cannot generate a desirable vibration operation, and a vibration sensitivity of the vibration speaker deteriorates.
The conventional vibration speaker is disadvantageous in that an effective space is limited for the vibrator to move upward and downward since the vibration speaker becomes slim, an unstable vibration occurs due to contact between the vibrator and lower and upper surfaces of the case 100, thereby generating noise and reducing a life-span of the vibration speaker.